In the EP 0820210 A2 a method and apparatus for beam forming of the microphone characteristic has been disclosed, by which a pre-determined characteristic of amplification in dependency of the direction from which acoustical signals are received at two spaced apart microphones is formed in that repetitevely a mutual delay signal is determined from the output signals of the microphones and according to the reception delay of the microphones, one of the output signals is filtered, thereby the filtering transfer characteristic is controlled in dependency of the mutual delay signal. The output signal of the filtering is exploited as electrical reception signal.
Thus, in principle the time delay or phase lag between the two output signals of the two microphones is used for a beam forming operation.
In a digital hearing aid the single samples are taken with a time difference equally divided by the sampling frequency, f.i. normally 32 usec. The desired delay between two or more microphone signals are typically less than 32 usec, e.g. 15 usec. A way to obtain a delay which is less than one sample is to have the DSP interpolate signal values between two samples with a certain delay and use those delayed sample values in the further processing. But this requires many calculations and takes up valuable space and power in the DSP.
Also, the signal will be somewhat distorted as the delyed samples are not xe2x80x9ctruexe2x80x9d samples.
However, for an active control of beam forming properties in a directional hearing aid, the delays that could be realized, based on the sample frequency and conventional shift register technology would be much too long to be useful.
In order to realize sample delays as low as 1 usec the conventional technology can not be used.
Thus, it is an object of the present invention to create a novel hearing aid with beam forming properties in which an active control of the delay of at least one of the incoming signals of a hearing aid having at least two microphones can be used for active beam forming. With such a hearing aid a great number of various directional orientations of hearing aids could actively and controllably be realized.
Particularly, by using faster sampling rates, the samples because of their shorter time intervals could be used directly, so that desirable short delays could be realized.
By using a sigma-delta converter with a sampling rate or clock frequency of f.i. 1 MHz and by inserting a 1 bit adjustable and controllable digital delay line in the bit stream from one of the sigma-delta converters to the corresponding decimator filter of the converter one could obtain delayed difference steps of multiples of 1 usec, which could not be achieved with conventional hearing aid technology.
In connection with a single channel hearing aid, it is known per se from German published patent application No. 44 41 996 to use a high-frequency-clocked signal-delta converter as part of an advantageous amplifier stage.
For this purpose a new hearing aid with beam forming properties has been developed, which has at least two microphone channels for at least two microphones, said microphone channels containing each an analog to digital converter, and having at least one programmable or programmed digital signal processor, as well as a digital to anlalog converter, at least one receiver and a battery for power supply.
This new hearing aid, in accordance with the present invention, contains in each of said microphone channels a sigma-delta-type analog to digital converter including a digital low pass filter and decimator filter for converting a 1 bit stream of a high clock frequency into a digital word sequence of a lower clock frequency, whereby at least one of said at least two microphone channels contains a controllable delay device connected to the input side of the respective digital low pass filter and decimator filter of said channel, said delay device being controllable by said at least one digital signal processor.
It is advantagous to have said delay device integrated into the sigma-delta ADC.
It is of particular importance to use, as a delay device, a programmable or program controlled tapped shift register for realizing various different delays of the bit stream signals before their entering the respective digital low pass filter and decimator. In order to realize controllable delays as short as 1 usec it is of advantage to use a clock frequency for the sigma delta ADC in the range of 1 MHz or even higher and a clock frequency in the area of 10 to 50 kHz for the digital low pass filter and decimator filter.
It is now obvious that with such a configuration of the input side of a beam forming hearing aid with active beam control various additional possibilities exist which are subject of the remaining claims. Particularly, by this new hearing aid a very high resolution delay may be achieved.